Solar cells are electronic devices capable of converting electromagnetic energy—such as the solar radiation—into electricity.
Such electronic devices are mainly comprised of semiconductor materials, which are characterized by solid crystalline structures having forbidden energy bands (“band gaps”) located between the valence bands and the conduction bands. A band gap defines an energy interval which is normally unavailable to free electrons. However, when solar radiation hits a material of such type in a solar cell, the electrons that occupy lower energy bands may be excited to the point of making an energetic jump and exceeding the band gap, for reaching higher energy bands. For example, when electrons in the valence bands of a semiconductor absorb sufficient energy from the photons of the incident solar radiation, such electrons may exceed the band gap and reach the conduction band.
Reaching the higher energy bands, such electrons leave empty locations within the lower energy bands; such empty locations, often referred to as “holes”, may move from atom to atom in the crystalline reticule. The holes act thus as charge carriers, in the same way as the free electrons in the conduction band, and contribute to the conductivity of the crystal.
In other words, each photon absorbed by the semiconductor generates a corresponding hole-electron pair. The set of electron-hole pairs formed by the photons absorption generates the so-called photocurrent of the solar cell. The holes and the electrons generated in this way may recombine with each others, subtracting their contribution to the maintenance of the photocurrent. In order to avoid (or at least to reduce as much as possible) this phenomenon for increasing the efficiency of the solar cell, a local electric field is generated within the semiconductor material. In this way, the holes and the electrons generated further to the absorption of the photons are accelerated by the local electric field toward opposite directions, and thus the probability that they will recombine before reaching the terminals of the solar cell drastically diminishes. Particularly, such electric field may be generated by means of the generation of a spatial charge region, such as the depletion region obtainable by means of a pn junction between a pair of oppositely doped semiconductor materials.
Such solar cells, generally used for space and terrestrial applications, may be of the single pn or np junction-type, or single-junction solar cells, or may be of the more-than-one pn or np junction-type, or multi-junction solar cells.
Single-junction solar cells are substantially constituted by the presence of a single pn or np junction. Conversely, multi-junction solar cells are implemented by stacking various pn or np junctions, presently from two to five junctions. The different junctions are made in different semiconductor materials, and are electrically coupled to each other in series by means of tunnel diodes interposed between each pair of adjacent junctions.
Each of the different superimposed junctions forms a so-called elementary cell, and the various elementary cells are capable of singularly converting the various portions of the incident solar radiation spectrum in a more efficient way compared to the one obtainable with a single junction.
Multi-junction cells may have the advantage of being capable of providing a higher output voltage with respect to single-junction cells, the overall voltage being equal to the sum of the voltages of the single elementary cells (minus a little voltage drop in the tunnel diodes serially coupling the cells).
In order to be manufactured, the various material layers directed to form the different junctions are typically obtained with an epitaxial growth technique through deposition (for example by means of the Metal Organic Chemical Vapor Deposition technique, or MOCVD) on commercial germanium (Ge), silicon (Si), or gallium arsenide (GaAs) substrates.
During the last few years, the performances of solar cells based on compounds of elements of Groups III and V of the periodic table of the elements, i.e. based on the III-V compounds, and in particular of GaAs solar cells, have been increasing, thanks to the progress of technologies that allow developing new materials for manufacturing three, four, and also five junction cells.
The cost of a multi-junction solar cell is slightly higher than the cost of a single-junction one, and its efficiency is significantly higher (in a condition of out-of-terrestrial-atmosphere illumination at 25° C., the efficiency is approximately equal to 28% for a triple-junction cell, compared to 20% for a single-junction one); for this reason, especially for aerospace applications, the market is oriented toward the use of these new, more efficient, devices. For example, the present big telecommunication satellites may require the use of triple-junction solar cells. These cells have, on the other hand, an employ in terrestrial applications, such as in optical concentration systems.
As already mentioned above, the efficiency of a solar cell strictly depends on the recombination phenomenon of the photo-generated hole-electron pairs. The hole-electron pairs generated outside of the depletion region may not be subjected to the action of any electric field, and thus may have a high probability of recombining, subtracting their contributions from the photo-generated current.
In order to improve the efficiency of a solar cell, a known technique provides for increasing the depth of the depletion region (and, thus, increasing the portion of the semiconductor material that is subjected to the electric field) by inserting a portion of an intrinsic (i.e., that is not doped) semiconductor material between the n-doped portion and the p-doped portion. This technique generally increases the efficiency of the solar cell, until the thickness of the depletion region reaches a value such as to limit the output voltage. Beyond this value, the efficiency of the device starts to diminish.
According to another technique, the semiconductor material portions of the solar cell that do not belong to the depletion layer may be subjected to local electric fields obtained by means of the use of proper doping gradients. Particularly, in “Semiconductors and Semimetals, Vol. II”, by Harold J. Hovel, the semiconductor portion of the p type is doped according to a linear doping gradient. Thanks to the presence of the linear doping gradient, the semiconductor material portion of the p type may be subjected to a constant electric field capable of accelerating the hole-electron pairs that are generated therein but outside of the depletion region.